Dimorphic Fungi

Slow growing fungi that grow as molds at 25-30° and yeast at 35-37°
(human body temperature)
The commonly considered dimorphic fungi include:

Sporothrix schenckii

Histoplasma capsulatum

Blastomyces dermatitidis

Paracoccidioides brasiliensis

Coccidioides immitis

Penicillium marneffei

Of special concern, thermally dimorphic fungi can be dangerous to culture
in routine microbiology laboratory settings. Some laboratories recommend
transferring young mold colonies from petri dishes to slants as soon as they appear,
limiting the chance of Coccidioides arthrospore formation and laboratory
worker exposure should C. immitis be cultured (Al-Doory 1980). The slow
growth and nonspecific gross appearance of these fungi complicate diagnosis.
The following fungi may be confused:

Coccidioides immitis

Colonies grow moderately rapidly

Mold colonies on 5% sheep blood agar are typically gray-white,
and have a delicate, hair-like texture; on Sabouraud. dextrose agar colonies
evolve from moist, membranous gray appearance to white and cottony
with the appearance of aerial mycelium. Colonies often darken with age.
The reverse is gray or white

Yeast colonies (35-37°) may take several generations to obtain and appear moist
and white on brain heart infusion agar

Cyclohexamide inhibits yeast forms

In tissue, large, irregularly sized thick walled spherules ranging in size
from 10-50 μ to as much as 10-200 μ are characteristic

Spherules contain small, round endospores (2-4 μ) which may be
confused with Histoplasma

FIG. 1. High power photomicrograph of
Coccidioides immitis in lung tissue, H&E.

Sporothrix schenckii

Subcutaneous mycosis; found world-wide

Rose gardener's disease

Mold colonies
mature rapidly and are white to yellow, wrinkling and folding with maturation

Microscopically hyphae are delicate with small, unicellular, oval to round
microconidia (2-6μ) developing laterally along hyphae in a sleeve-like pattern

Yeast forms vary in size and shape but may be round, oval, and fusiform; budding forms
are seen

FIG. 1.Sporothrix in slide culture. Note the sleeve-like way that microconidia cling to the hyphae.

Penicillium marneffei

Endemic in southeast Asia, P. marneffei is known to cause a wide
variety of disease among immunocompromised and, less frequently, immunocompetent
patients.

Hyphal colonies appear rapidly and white-tan, velvety, and flat, although
distinct spicules of growth may protrude from the surface of the culture.
Red soluble pigment is noted in maturing colonies, although this finding
is not specific for P. marneffei.

The mold form (grown at 25-30°) resembles an otherwise typical Penicillium,
with septate hyphae and smooth generally smooth conidia aloft phialides which
in turn are borne by metulae. The conidiophore typically carries 4-5 metulae,
each of which bears 4-6 phialides.

The yeast form (grown at 35-37°) are round to oval and are 3-7μ in diameter; yeast
reproduce by fission rather than budding.